Pre-incubation feeding activities and energy budgets of Snow Geese: can food on the breeding grounds influence fecundity?

The potential contribution of early spring feeding conditions in the Arctic to clutch size variation was examined in a population of Lesser Snow Geese Anser caerulescens caerulescens. Behavioural observations were combined with energetic analyses of food material to derive an estimate of the energy budgets of pre-laying and laying females. Food intake of females between arrival on the breeding grounds and incubation was considerable; estimated energy gains in this period were in the same magnitude as the cost of one or several eggs. The pre-laying period spent on the breeding grounds can thus be energetically beneficial rather than costly. Accumulation of resources for reproduction in Snow Geese is a continual process including the breeding grounds, and nutrient limitation after arrival in the Arctic cannot sufficiently explain the environmental component of clutch size variation. The timing of migration and follicle development is such that clutch size decisions are sometimes made during the late stages of migration and some-times after arrival. In the latter case food conditions on the breeding grounds may greatly influence clutch size; in the former case they may still influence readjustments of clutch size after the initial decision. The universal negative correlation between clutch size and laying date in Snow Geese can be explained by negative fitness consequences of late hatching, which outweigh the benefits of delayed laying and further nutrient accumulation. Food shortage on the breeding grounds may sometimes be a secondary factor contributing to seasonal clutch size decline.

FREQUENCY-DEPENDENT FITNESS CONSEQUENCES OF INTRASPECIFIC NEST PARASITISM IN SNOW GEESE.

The reproductive efficiency, defined as the number of breeding recruits produced per egg laid; of intraspecific nest parasites; of hosts in parasitized nests; and of unparasitized nesting females, was measured for 14 years for lesser snow geese Anser caerulescens caerulescens nesting near Churchill, Manitoba, Canada. Relative efficiencies were 0.71-0.88, 0.91, and 1.0 for eggs of parasites, hosts, and unparasitized birds, respectively. Differences in the hatching probabilities of the three classes of eggs produced the efficiency differences. Parasitic success was limited by the parasites' failure to place more eggs than expected by chance into nests at the appropriate time relative to host incubation. Host nesting success was lower when more than one parasitic egg was added to the clutch. No differences in gosling survival and breeding recruitment probabilities were detected among any categories of goslings. Thus, hatching parasitic young are at no disadvantage relative to parental young, and there is no support for the hypothesis that increased success of host young at later stages of reproduction might offset negative effects at the egg stage. The hatching efficiency of parasitic eggs declined more rapidly than that of parental eggs as the parasitism rate increased. Efficiencies were similar when 3-4% of the eggs laid per year were parasitic, but relative parasitic efficiency was significantly lower when the parasitism rate was 8-9% or more. Using ancillary information and assumptions about the fecundity, viability, and behavioral flexibility of parasitic and parental females, we conclude that intraspecific nest parasitism could compete with nesting as a reproductive strategy in this population. The conditional use of parasitism by a large component of the population in certain years, however, combined with negative-frequency dependent success, limits the potential spread of a purely parasitic strategy in this population.

Intra-seasonal decline of clutch size in Lesser Snow Geese.

Different mechanisms proposed to explain the intra-seasonal decline in clutch size of Lesser Snow Geese (Anser caerulescens caerulescens) were tested at the La Perouse Bay colony, Manitoba, Canada. Ovary examination of females collected after laying revealed that the actual number of eggs produced per female decreased over the laying period. This finding eliminates nest-parasitism, partial clutch predation and renesting attempts as sufficient explanations for seasonal clutch size decline. Follicular atresia induced by reserve depletion was also rejected since its occurrence was similar among early and late nesters. The decline in clutch size was observed within age-classes, and therefore age effects on clutch size and laying date per se do not account for the observed relationship. Clutch size and laying date were respectively positively and negatively correlated with the amount of nutrient reserves in females at the onset of laying, and also covaried within individuals observed breeding in several seasons. Laying date repeatability was estimated at 0.22. It is postulated that the seasonal decline in clutch size results mainly from a positive feed-back of female's nutrient reserves on the hormones controlling ovary development. Hatching synchrony or shortness of the breeding season may be the ultimate factors responsible for the intra-seasonal clutch size decline in Lesser Snow Geese.

LIFE HISTORY STUDIES OF THE LESSER SNOW GOOSE (ANSER CAERULESCENS CAERULESCENS). VI. PLUMAGE POLYMORPHISM, ASSORTATIVE MATING AND FITNESS.

The investigated the hypothesis that positive assortative mating for plumage coloration observed in populations of the dichromatic Lesser Snow Goose (Anser caerulescens caerulescens) evolved and persists due to selective advantages accruing to individuals choosing mates phenotypically similar to themselves. We examined potential differences between pure (white × white, blue × blue) and mixed (white × blue, blue × white) pairs for an array of fitness components related to both fecundity and viability. While no differences were detected for most components, mixed pairs consistently enjoyed enhanced nesting success relative to their pure counterparts. In addition, pre-reproductive viability and female offspring recruitment were significantly greater for mixed pairs for some of the cohorts examined. Not only have we failed to find enhanced reproductive success associated with positive assortment, we have provided evidence that there is some advantage associated with negative assortment. In light of our findings, we suggest: 1) that positive assortment results from the use of familial color as one element in species recognition; 2) that the enhanced fitness of mixed pairs, particularly with respect to nesting success, results from complementation of parental behavior; and 3) that while negative assortment will not likely become the rule in this population, the selective advantage of mixed pairs is a potential determinant of pair type frequencies in the La Pérouse Bay population.

LIFE HISTORY STUDIES OF THE LESSER SNOW GOOSE (ANSER CAERULESCENS CAERULESCENS). IV. THE SELECTIVE VALUE OF PLUMAGE POLYMORPHISM: NET VIABILITY, THE TIMING OF MATURATION, AND BREEDING PROPENSITY.

Cooch (1961, 1963) suggested that changes in the genotypic composition of snow goose colonies comprising the Hudson Bay-Foxe Basin population could be attributed to selection favoring the blue phenotype. In the preceding paper (Cooke et al., 1985), we examined potential differences in net fecundity between the two phenotypes at La Pérouse Bay in northern Manitoba. No substantial differences in any component of fecundity were detected. In the present paper, we examined potential differences in pre-reproductive and adult viability, age of maturation, and breeding propensity. Again, no differences associated with the plumage polymorphism were detected in any of these life history characteristics. Thus, despite a thorough analysis of the complete life cycle, we were unable to uncover any evidence supporting a selection hypothesis for the persistence and dynamics of this conspicuous polymorphism. An alternate hypothesis based primarily on gene flow and assortative mating appears more plausible.

LIFE HISTORY STUDIES OF THE LESSER SNOW GOOSE (ANSER CAERULESCENS CAERULESCENS). III. THE SELECTIVE VALUE OF PLUMAGE POLYMORPHISM: NET FECUNDITY.

Between 1969 and 1977 the frequency of the blue phenotype of the dimorphic Lesser Snow Goose (Anser caerulescens caerulescens) showed a steady increase at the La Pérouse Bay colony near Churchill, Manitoba. Cooch (1961, 1963) suggested the global increase resulted from selection pressures favoring blue individuals. The selection hypothesis was evaluated by examining phenotypic differences in net fecundity. We partitioned the reproductive cycle into a series of stages, each defined by a particular index of fecundity. Despite large samples we were unable to detect any significant differences between the two maternal phenotypes in those indices that could conceivably influence population dynamics. We cannot, however, dismiss selection as the mechanism of population change, nor as a contributor to the maintenance of the polymorphism without assessing potential phenotypic differences in viability, age of maturation, and breeding propensity. These attributes are examined in the following paper (Rockwell et al., 1985).

Life history studies of the Lesser Snow Goose (Anser caerulescens caerulescens) : I. The influence of age and time on fecundity.

In many species, fecundity is dependent on age. For some populations - particularly those exploiting temporally variable environments - these age-specific fecundities may themselves vary with time. In this report, we investigate the general effects of age and time on clutch size in a wild population of Lesser Snow Geese (Anser caerulescens caerulescens). We show that both factors affect clutch size independently. Age effects are confined almost exclusively to the two, three and four year old age classes. Mean clutch sizes of all age classes show considerable inter-seasonal variation. These temporal differences in fecundity will ultimately influence the dynamics of the population. In particular, they imply that there will be no convergence to a constant growth rate or a stable age distribution.